THE LAW OF PERIODICITY OR RHYTHM IN NATURE. 89 
the laws of rhythm in disease. Certain fevers have their regu- 
lar periods of attack, as intermittent fever; while all diseases 
have their periods of exacerbation, however invariable the 
symptoms may seem to be to the ordinary observer or even to 
the patient himself. 
Doubtless the fact that certain hereditary diseases do not 
appear in the offspring at once, but only at the age at which 
they were manifested in the parents, is owing to the same 
cause. : 
Let us now examine more thoroughly into the real nature of 
this rhythm which pervades the entire universe. 
If a bow be drawn across a violin-string on which some small 
pieces of paper have been placed, these will be seen to fly off; 
and if the largest string be experimented upon, it can be ob- 
served to be in rapid to-and-fro motion, known as vibration, 
which motion is perfectly regular, a definite number of move- 
ments occurring within a measured period of time; in other 
words the motion is rhythmical. In strings of the finest size 
the motion is not visible, but we judge of its existence because 
of the result, which is in each instance a sound. Sound is to us, 
however, an affection of the nerve of hearing and the brain, 
owing to the vibrations of the ear caused by similar vibra- 
tions of the violin-strings. The movements of the nerves and 
nerve-cells are invisible and molecular, and we seem to be 
justified in regarding molecular movements as constant and 
associated with all the properties of matter whether living or 
dead. 
We see, then, that all things living and lifeless are in con- 
stant motion, visible or invisible; there is no such thing in the 
universe as stable equilibrium. Change, ceaseless change, is 
written on all things; and, so far as we can judge, these 
changes, on the whole, tend to higher development. Neither 
rhythm, however, nor anything else, is perfect. Even the mo- 
tions of planets are subject to perturbations or irregularities 
in their periodicity. This subject is plainly boundless in .its 
scope. We have introduced it at this stage to prepare for its 
study in detail in dealing with each function of the animal 
body. If we are correct as to the universality of the law of 
rhythm, its importance in biology deserves fuller recognition 
than it has yet received in works on physiology; it will, ac- 
cordingly, be frequently referred to in the future chapters of 
this book. ; 
